Page 7 - 6273

The mass of the atom, expressed in an atomic number, is called the relative

atomic mass (Ar). It is dimensionless. The atomic masses of chemical elements

are indicated in the periodic system of elements.

Molecular mass (Mr) is the mass of a molecule, expressed in atomic mass

units. It is also dimensionless. The molecular masses of compounds are equal to

the sum of the masses of the atoms forming the molecule.

For example, the atomic mass of the Hydrogen, Ar (H) = 1, Ar (Be) = 9, Ar

(O) = 16, Ar (Mg) = 24, respectively, the mass of hydrogen molecule consisting of

two atoms of the Hydrogen, Mr (H 2) = 2 · 1 = 2, Similarly, Mr (N 2) = 2 · 14 = 28,

Mr (CO 2) = 12 + 2 · 16 = 44, Mr (H 2SO 4) = 2 · 1 + 32 + 4 · 16 = 98.

For example, the relative atomic mass of Magnesium is 24. This means that

Magnesium atom is "heavier" from the Hydrogen atom in 24/1 = 24 times, from

the hydrogen molecule at 24/2 = 12 times, from the Carbon atom at 24/12 - twice,
from the Oxygen atom at 24/16 = 1,5 times, from the sulfur atom at 24/32 = 0,75

times, that is, the "lighter" at 32/24 = 1,33 times. In chemistry a physical quantity
is used,

A mole is a quantity of substance that contains as many structural particle
12
entities as there are atoms in 12 g of the isotope C

which is called the amount of substance (ν). The measurement unit of the

substance amount is mole.

It has been experimentally found that 12 g of Carbon 12C isotope contains
23
6.02 ·10 atoms. Consequently, 1 mole of any substance contains 6.02 · 10 23

structural particles (atoms, molecules, ions, etc.). This number is called the

Avogadro (Na)

The mass of 1 mole of the substance is called the molar mass of the

substance (M). The molar mass of a substance is numerically equal to the relative

atomic or molecular mass; unit of dimension - g/mol.

7